someone that is 'heaven on a stick' is very attractive.
The meaning probably comes from the small ice-cream blocks or sweets that you buy on a small stick and are 'heavenly' in taste.
There *can* be a light sexual meaning to it when you use this expression to describe a person. 'He/she is heaven on a stick' means you think the person is very attractive. But the expression is generally quite polite and not vulgar at all. You are just saying you wish you could 'sample' the person in a joking, very light manner.